3 Internet Service Providers What is an ISP? An ISP is an organization that connects business or residential customers to Internet (backbone). An Internet Service Provider (ISP) is a company that provides access to the Internet. Their customers can be businesses, individuals or organizations. The advent of ISPs has made connecting to the Internet an affordable and convenient option for general people Internet structure is roughly hierarchical In the public Internet, access networks situated at the edge of the Internet are connected to the rest of the Internet through a tiered hierarchy of Internet Service Providers (ISPs) 3

4 ISP Categories ISP Categories Tier-1 ISPs (Internet Backbone) Tier-2 ISPs Tier-3 ISPs Backbone Providers / Tier-1 ISPs These ISPs are nationwide or multinational organizations that control Internet routing. They often own significant pieces of backbone itself National Providers / Tier-2 ISPs These ISPs buy capacity (bandwidth) and routing services from backbone providers and run Points Of Presence (POP: location of access points to the Internet) across the country. Local Providers / Tier-3 ISPs These ISPs operate in the same way as the national ISPs, but on a smaller geographical area 4

5 Points of Presence (POPs) POPs are private peering points of ISPs Within an ISPs network, the physical location / points at which the ISP connect to other ISPs are known as Points of Presence (POPs) A POP is simply a group of one or more routers in the ISP s network at which routers in other ISPs can connect. The POP is in the ISP s switch site or in a colocation space, the contents will always contain access equipment and an IP router. At the core of the POP is a router that acts as the central hub for routing within the POP and is also used to terminate high capacity connections. 5

6 Network Access Points (NAPs) NAPs are public peering points of ISPs When two ISPs are directly connected to each other, they are said to peer with each other. The NAP can be owned and operated by either some thirdparty telecommunications company or by an Internet backbone provider. NAPs exchange huge quantities of traffic among many ISPs Often a NAPs uses high speed ATM switching technology, with IP running on the top of ATM 6

7 Backbone Providers / Tier-1 ISPs Tier-1 ISPs Also known as Internet Backbone Exists at the center of the Internet Architecture Directly connected to each of the other tier-1 ISPs Connected to a large number of tier-2 ISPs and other customer networks International in coverage Two tier-1 ISPs can also peer with each other by connecting together a pair of POPs, one from each of the two ISPs. The trend is for the tier-1 ISPs to interconnect with each other directly at private peering points. Examples (e.g., UUNet, BBN/Genuity, Sprint, AT&T) 7

9 National Providers / Tier-2 ISPs Tier-2 ISPs Provides smaller coverage as compared to tier-1 ISPs National Coverage Connect to one or more tier-1 ISPs Connect to other tier-2 ISPs as well. Tier-2 ISPs typically have regional or national coverage and connects only to a few of tier-1 ISPs A tier-2 ISP is said to be a customer of the tier-1 ISP to which it is connected, and the tier-1 ISP is said to be a provider to its customer. The trend for tier-2 ISPs is to interconnect with other tier-2 ISPs and with tier-1 ISPs at NAPs 9

11 Local Providers / Tier-3 ISPs Tier-3 ISPs last hop ( access ) network (closest to end systems) Local Coverage Below tier-2 ISPs are the lower-tier ISPs, which connect to the larger Internet via one or more tier-2 ISPs Users and content providers are the customers of lower-tier ISPs and lower-tier ISPs are the customers of higher-tier ISPs 11

15 Delay Packet Switched Networks Considering what can happen to a packet as it travels from its source to its destination. As a packet travels from one node to other node (host or end system), it suffers from several types of delays at each node along the path Most important types of delays are: Processing Delay Queuing Delay Transmission Delay Propagation Delay 15

16 Types of Delay Processing Delay The time required to process (examine the packet s header and determine where to direct the packet) is part of the processing delay Processing delay in high-speed routers is typically on the order of microseconds or less. After this nodal processing, the router directs the packet to the queue that precedes the link to the next router. Processing Delay depends on the processing speed of a router. 16

17 Types of Delay Queuing Delay At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link. The queuing delay of a packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission across the link If queue is empty, and no other packet is being transmitted, the queuing delay will be zero If traffic is heavy and many other packets are waiting to be transmitted, the queuing delay will be long Thus, queuing delay depends on the intensity and nature of traffic arriving at the queue. Queuing delays can be in the order of microseconds to milliseconds in practice 17

18 Types of Delay Transmission Delay It is the amount of time required to push an entire packet into the link The time taken by a transmitter to send out all the bits of a packet onto the medium Also called Store and Forward Delay Node receives complete packet before forwarding Transmission Delay is directly proportional to the length of the packet Transmission delays are typically in the order of microseconds to milliseconds in practice 18

20 Types of Delay Propagation Delay Time it takes a bit to propagate from one node to the next. The time required by a bit to propagate from the beginning of the link to the next router is called propagation delay The bit propagates at the propagation speed of the link which depends on the physical medium being used. It is typically in the range of: 2 x 10 8 meters/sec to 3 x 10 8 meters/second In wide area networks, propagation delays are on the order of milliseconds 20

23 Queuing Delay Queuing delay is most complicated and interested delay as compared to other components of nodal delay (processing, transmission, propagation) Queuing delay can vary from packet to packet Example: if ten packets arrive at an empty queue, the first packet will suffer no queuing delay while the last packet will suffer large queuing delay 23

24 Queuing Delay Queuing delay depends on: Average Rate at which the packets arrives at a queue (a = packets/sec) Transmission Rate of the link (R = bits/sec) Nature of the incoming traffic (bursty/periodic) Assume that all the packets are of equal length say L bits Then the average rate at which the bits arrive at the queue will be La bits/sec Traffic Intensity = La/R This ratio helps in estimating the extent of queuing delay 24

25 Traffic Intensity Traffic Intensity If La/R is > 1 It means that the average rate at which the bits arrive at the queue exceeds the rate at which the bits can be transmitted from the queue. In this undesirable situation, the queue will tend to increase without bound and the queuing delay will reach to infinity! A golden rule in traffic engineering Desing your systems so that the traffic intensity is no greater than 1s 25

26 Traffic Intensity Traffic Intensity If La/R is > 1 If the traffic intensity is close to one, there will be intervals of time when the arrival rate exceeds the transmission capacity and a queue will form As the traffic intensity approaches 1, the average queue length gets larger and larger If La/R is < 1 If the traffic intensity is close to zero, then the packets arrivals are few and far between, and it is unlikely that an arriving packet will find another packet in the queue Average queuing delay will be close to zero 26

29 Packet Loss In reality a queue has a finite capacity As the traffic intensity approaches 1, a packet can arrive to find a full queue. With no place to store such a packet, a router will drop that packet; that is the packet will be lost The fraction of lost packets increases as the traffic intensity increases Thus, a node performance also includes the probability of packet loss A lost packet may be retransmitted on an end-to-end basis, either the application or transport layer protocol. 29

30 End-to-End Delay The total delay from source to destination is referred to as end-to-end delay Example: Suppose that the queuing delay is negligible as the network is uncongested, then the end-to-end delay between the source and destination having N-1 routers in between will be: d end-end = N (d proc + d trans +d prop ) 30

31 Delays and Routes in the Internet Traceroute A program that sends multiple special packets towards the destination As these packets work their way towards the destination, they pass through a series of routers. When a router receives one of these special packets, it sends a short message back to the source. This message contains the name and address of the router For Details: Consult Traceroute: RFC 1393 To Do: Explore the Netstat utility 31

33 Layered Architecture Design Philosophy of Layered Architecture The complex task of communication is broken into simpler sub-tasks or modules Each layer performs a subset of the required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers Helps in troubleshooting and identifying the problem 33

39 History of Internet In 1960s the telephone network was the worlds most dominant communication network Uses Circuit switching which is appropriate for voice traffic by supporting constant data rates With the increasing importance of computers, the need for interconnecting different geographically dispersed computers was realized. Three research groups laid the foundations of packet switching notion for computers communications: MIT (Leonard Kleinrock) Rand Institute (Paul Baran) National Physical Laboratory (NPL) 39

40 History of Internet Idea of Packet Switching Principles of Packet Switching were conceived in 1957 by Paul Baran and others First Paper by him on Packet Switching First Book on Internet in which Idea of Packet Switching was declared more efficient than Circuit Switching Paul Baran used first time Digital Computer Technology for Communication between Switching Networks and divided the data into Message Blocks and reassembled at destination with some error detection technique Dynamic Routing of these Message Blocks was also proposed by Baran First Packet Switching Network was designed and Implemented 40

41 The Internet s Infancy: 1960s DARPA (Defense Advanced Research Project Agency) was established as an outcome of the Sputnik1 launch in 1957 by NASA (National Aeronautics and Space Administration), formally known as ARPA Computers in the form of Network was visualized and Implemented for data communication by Taylor First Wide Area Computer Network was developed First Packet Switching Router in the form of IMP (Interface Message Processor) was proposed; about a size of refrigerator BBN designed IMPs and established the protocols allowing IMPs to communicate with each other. 41

42 The Internet s Infancy: 1960s Network Working Group (NWG) was formed to ensure the stability of communication protocols. Steve Crocker wrote first minutes of meetings IMP1: The first node of the ARPANET The IMPs (Interface Message Processors) connected both host computers and other IMPs and functioned to: Receive data Check for errors Retransmit, if error exists Route the packets Verify that packet are sent to intended receivers 42

43 The Internet s Infancy: 1960s This documents was called RFC (Request for Comments) to take suggestions from peoples; later it became a Standard NWG designed first host-to-host protocols for host to IMP and computer to computer communication Device Drivers were proposed to enable communication between different operating systems and hardware The destination IMPs used hop-by-hop acknowledgements. Since the source systems were different, so a software had to be designed to enable them to communicate, which is called a device driver 43

44 The Internet Early Years: 1970s NCP (Network Control Protocol) was designed; used Stop and Wait flow control.it was the first host-tohost communication protocol that is used between the ARPANET end systems Idea of Open-Architecture Network was floated TCP (Transmission Control Flow Control) was designed for data transmission and Checksum was used for error detection 44

46 The Internet Growth Begins: s Ethernet was proposed as a LAN Technology First Ethernet protocol was developed IP was proposed for Addressing purposes TCP/IP Protocol Suite was designed UNET: First TCP/IP product was introduced for Ethernet BSD (Berkeley Software Division) Unix Operating System was introduced 1 st January It was decided to replace NCP to TCP/IP for all Networks that gives birth to INTERNET FTP, SMTP, DNS were introduced 46

47 The Internet Growth Begins: 1980s UDP comes into play for Real time Applications like Voice and Video USENET modified for Newsgroups All Super Computers were connected to form a Backbone Network called NSFNET which started from 56Kbps and in 1988 was converted to T1 Line I.e., 1.544Mbps First Internet Worm was invaded effecting around 60,000 Hosts WWW was created by Berners-Lee who also created First Web Server and Browser (Also designed HTTP later) Clinton received first at First Real Web Browser called MOSAIC was introduced 47

48 Internet Privatization: 1990s E-business started at Internet NSFNET decided to Privatize Internet by creating 4 NAPs (Network Access Points) and giving permission to ISPs to connect to NAPs NSF Created High Speed Backbone Network Service to provide high-bandwidth connectivity (155 to 622Mbps) among NSF s SCCs (Super Computer Centers) Internet2 was Created by Connecting all Top 100 Universities to these SCCs via GigaPOPs (Gigabits point of presence) Internet2: It is Hybrid Network whose Members are Major Universities and Research Organizations. Several Access Speed Transitions from 56Kbps Modem to ISDN (64-128Kbps), DSL Asymmetric Service to Cable Modems etc. 48

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